Logarithmic receiver device which compensates for received signal strength

ABSTRACT

A logarithmic amplifier for automatically compensating for nonlinear input detectors having a voltage divider network in combination with a plurality of nonlinearly operated transistors. Particularly, the transistors have a control terminal connected to the voltage divider network for receiving logarithmic portions of the input voltage and a common output terminal for summing the outputs of the transistors.

United States Patent 1191 Chow et al. 1 1March 13, 1973 541 LOGARITHMICRECEIVER DEVICE 2,879,385 3/1959 1.11m; ..328/l45 wmc COMPENSATES F033.2111 14: 11/1965 Newhousc 1 =11. .143/1 1415 RECEIVED SIGNAL STRENGTHInventors: Henry Chow, Detroit; William R.

Faris, Berkeley; Howard F. Lynn, Livonia; Delmar V. Payne, Ferndale, allof Mich.

The Bendix Corporation, Southfield, Mich.

Filed: Dec. 15, 1966 Appl. No.: 602,456

Assignee:

Related U.S. Application Data FOREIGN PATENTS OR APPLICATIONS 915,2714/1959 Great Britain ..328/145 Primary Examiner-Malcolm F. HublerAttorney Richard T. Seeger, Vett Parsigian and Plante, Arens, Hartz &O'Brien ABSTRACT U-S. Cl. voltage divider network for receivinglogarithmic po 343/114-5 tions of the input voltage and a common outputter- [51] Int; Cl ..G0ls 3/16, G06g 7/24 minai for Summing the (mtputs fthe transistors [58] Field of Search ..343/ll4.5, 119; 328/142-145;307/885, 229, 230

11 Claims, 12 Drawing Figures [56] References Cited UNITED STATESPATENTS 2,577,506 12/1951 Belleville ..328/l45 1 "5:512? 1 ,1, 32 ATTEN.-8 DIFFERENCE :2:

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54 as mmruo: INDCATOR SELECTOR -q SOUELCH POWER 1.:ve1. swlrcu Km AUDIOPATENTEDIIAR 1 3 I913 3, 720 .946

SHEET 2 or 5 Z 9 T U W l o s O l a E g 2a "1 z w a a 3. :1 V M an SINVENTORS HOWARD F. LYNN 8 y HENRY CHOW WILLIAM R. FARIS DELMAR V. PAYNEgp awrw ATTORNEY PATENTEDHAR 13 ms 3, 720.946

SHEET 3 BF 5 i a (DIRECT) INVENTORS HOWARD F. LYNN BY HENRY CHOW WILLlAMR. FARIS DELMAR V. PAYNE ATTORNEY 0 PAIENIEI] MARIE] I975 SHEET '4 OF 5AMP 94 CRYSTAL (102 BIAS LOG] EMITTER X108 AMP FOLLOYIER AMPL'F'ER 5mm:EMITTER EMITTER I I FOLLOWER FoLLowER' "FoLLowER DELAY (109 EMITTER X11080 82 34 96 FOLLOWER I L063 EMITTER X112 38 AMP FOLLOWER f9 TRANSISTOROUTPUT CUTOFF ATTENUATOR SHAFT I18 I I OSCILLOSCOPE 126 123T I SIGNALGENERATOR q a 0 i LOG-VIDEO AMPLIFIER c LOG-VIDEO AMPLIFIER INVENTORS103 HOWARD F. LYNN F/g. /0C7 HENRY CHOW BY WILLIAM R. FARIS TRANSISTORINPUT LE'VEL DELMAR v. PAYNE OFF POINT ATTO NE) PATENTEDHAR I 3 I975SHEET 5 OF 5 INVENTORS HOWARD F. LYNN HENRY CHOW WILLIAM R. PARIS DELMARV. PAYNE fi/W 0p ATTURNEYV LOGARITHMIC RECEIVER DEVICE WHICH COMPENSATESFOR RECEIVED SIGNAL STRENGTH This is a division of our copendingapplication Ser. No. 481,450, filed Aug. 19, 1965, which is acontinuation of application Ser. No. 1 18,195 filed June 19, 1961entitled Receiver Device, now abandoned.

This invention pertains to a receiver system which visually representson a display means the relative position of electromagnetic radiationsources such as radiofrequency generators. More particularly, thisinvention pertains to improvements of a receiving system adapted forplacement in a vehicle.

In these type receivers, the signals from the first pair of antennas arecompared and then sent to a first set of deflection plates of a cathoderay tube or other display device, and the signals from the second pairof antennas are compared and sent to a second set of deflection platesof the cathode ray tube. The signal from each pair or set of antennasdetermines the position of a spot on the cathode ray tube screen therebyindicating the position of the source of radiation.

However, as the radiation sources are approached, the signals becomestronger and the differences become larger. In previous systems, thiswould cause an error in the indication of the position of the sources onthe display means. This invention overcomes this problem by usingantenna patterns and detectors related so that the plot of thedifference between the logarithms of the two signals for each antennaset versus the angle of the received signal is linear. Logarithmicamplifiers are then used to amplify the detected signals which had beenreceived by the antennas and when the difference between the twologarithmic signals of each set of antennas is taken, a linear ratio isobtained which is used to position the representation of the angularposition of the radio-frequency generator or sources on the displaymeans. Regardless of how near or far the radiated source is, thedifference between the logarithms of the signals will always be the samebecause this is essentially a ratio between the two signals, andtherefore minimum error due to absolute amplitude variation of thesources is introduced.

In the system above described, it is a primary object to represent thereceived signals on the display so that there is an exact correlationbetween the position of the received signals as the antenna sees themand the position of the received signals on the display to provide alinear display. As mentioned, the logarithmic amplifiers in principleaccomplish this objective but in practice there are slight differencesbetween the four receiving systems, two in one plane and two in theorthogonal plane, and it is important that these differences beminimized. This invention accomplishes this by using a plurality oflog-taking circuits in each signal modifying means and using first andsecond attenuating means with each log-taking circuit to adjust theoutput of the signal modifying means resulting in a linear display. Thefirst attenuating means are placed on the input side of the signalmodifying means so that the incoming signal to each signal modifyingdevice is divided according to signal strength to the individuallog-taking members to get a desired composite log-taking function. Thesecond attenuating means in the output circuit of the individuallog-taking members further adjusts the outputs of the individuallog-taking members so that the overall logarithmic characteristic ofeach detector-amplifier is similar to the characteristics of the otherthree detector-amplifiers to a very high degree.

It is therefore an object of this invention to supply two attenuatingmeans to each signal modifying means which are located between theantenna and the sum and difference circuits of the monopulse receivingsystem with the first attenuating means dividing the input signal to aplurality of log-taking members according to levels of strength and thesecond attenuating means being connected to the output of each of thelogtaking means as a further adjustment tothe contribution of thelog-taking means to the overall logarithmic relation of the signalmodifying means so that the outputs of each of the four signal modifyingmeans are similar to a very high degree.

These and other objects will become more apparent when a preferredembodiment of this invention is described in connection with thedrawings in which:

FIG. 1 is a simplified block diagram of a preferred embodiment of thisinvention;

FIG. 2 is a partial perspective view of the antenna as it might bemounted on an airplane;

FIG. 3 is a partial sectioned view of an antenna mounted to a pyramidside;

FIG. 4 is a rectangular coordinate plot of an antenna pattern forlogarithmic periodic antennas;

FIG. 5 is a diagrammatic view of the antenna lobes aligned in onemeasurement plane;

FIG. 6 shows the relative outputs from a signal intercepting the lobesas shown in FIG. 5;

FIG. 7 shows an airplane receiving a radiation directly from aradio-frequency source and a reflected radiation;

FIG. 8 shows the waveform as received in the airplane from the directradiation and the reflected radiation;

FIG. 9 is a block diagram of each logarithmic amplifier;

FIG. 10 is a circuit diagram of each logarithmic amplifier;

FIG. 10a is a plot of signal-in and signal-out of an amplifying elementin a logarithmic amplifier; and

FIG. 11 is a schematic block diagram of a circuit used to align theamplifiers of this invention.

ANTENNA AND RECEIVER The antenna system and receiver used, which may beplaced in an airborne vehicle for receiving and presenting thecontinuous picture to the vehicle the position of radiation sources,emitting radio-frequency, will now be described with the aid of theschematic shown in FIG. 1 and with the diagrams of FIGS. 2-8. Located ina first alignment on the airplane are a first pair of complementaryantennas 32, 34 which are, in this embodiment, divergent so that theirlobes appear as in FIG. 5. The purpose of the two antennas 32 and 34,which are aligned along an axis, is to receive radio signals and thensend these signals to means, which will be later described, for locatingthe source position along that axis.

Also, located in the vehicle in a second alignment preferably orthogonalto the first alignment and similarly divergent, is a second pair ofcomplementary antennas 36, 38 which are connected to means, also laterdescribed, for determining the source positions along the axis of thesecond alignment.

Assume for the moment that lobe 32a (FIG. 5) is the lobe for antenna 32and lobe 34a is the lobe for antenna 34. As mentioned, these lobes aredirected outwardly at a divergent angle so that if a signal should bereceived at an angle such as signal 40, it will intercept lobe 320 atpoint X and intercept lobe 34a at point Y, giving corresponding levelsat voltages as shown in FIG. 6. By knowing the antenna patterns for eachantenna, and assuming the antennas are aligned along a horizontal axis,these voltages can be compared to determine horizontal component ofangle the received signal 40 makes with the center line 42. In a similarmanner, antennas 36 and 38 have characteristic lobes and assuming theyare aligned along a vertical axis, a signal which is received can bevertically located by comparing the voltages which are picked upsimultaneously by the antennas.

A particular antenna structure that gives satisfactory results is shownmounted to an aircraft nose in FIG. 2. Aircraft 26 has mounted in aforward position a truncated square pyramid 27 having four logarithmicperiodic antennas 27a, which would correspond to antennas 32 38, mountedon its sides. There are two sets of complementary antennas, withantennas on opposite sides of the pyramid 27 forming a set. Each antenna27a is connected to the center conductor of a coaxial line 29 whichextends through a pyramid side, FIG. 3, and is angled away from thepyramid side so that each antenna base 270 is spaced from the pyramidside. In this manner, the pyramid sides act as ground planes and antennapatterns such as those shown in FIG. 4 are possible.

In FIG. 4, two patterns for complementary antennas 27a are shown withReceived Signal Strength in decibels plotted along the ordinate andSignal Direction plotted along the abscissa. The curves intersect alongthe zero angle direction indicating that the signal strengths are equalat that point. At a signal direction a the curves are intersected atpoints a and 11 respectively and such direction would have a distinctiveset of curve intercepts which can be processed to indicate the signaldirection as explained below. The logarithmic periodic antenna known tothe art and described in 1957 IRE Convention Record, Part 1, pp. 1 19 to128 Broadband Logarithmically Periodic Antenna Structure by R. H.DuHamel and D. R. lsbell. This antenna has small side lobes andtherefore, is not ambiguous in that a signal intercepting one of theside lobes would not be mistaken for a signal intercepting the operatingrange of the antenna since any side lobe signal would be substantiallysmaller.

Each antenna, 32, 34, 36 and 38, is connected to a filter 32a 38a whichpass only those frequency ranges that are desired for a particularapplication. The signals are then passed through attenuators 32b 38bwhich are varied when received signal strengths reach a predeterminedlevel, in a manner described subsequently. Detectors 32c 38c areconnected between the attenuators 32b 38b and logarithm video frequencyamplifiers 32d 38d. The characteristics of the detectors are importantand must be matched with the antenna patterns and amplifiercharacteristics to provide a linear-logarithmic output at each amplifieroutput. In this embodiment, the detectors are of the crystal-video typewith approximate square law characteristics at low signal level changingto linear detector characteristics at high signal level. The radiofrequency portion of the received signal is filtered out by a low passfilter of the crystal construction.

The logarithm amplifiers 32d 38d receive the signals from the detectorsand convert them to a voltage which corresponds to a logarithm valuewhich when difierenced with a logarithm value of the signal in the otherantenna with which it is aligned, will determine accurately the positionof a radio-frequency source signal along the axis of the antenna pair.This difference will be in linear relation to the received signaldeviation from system boresight.

The two antennas of each pair of complementary antennas send to theirrespective crystal detectors a pair of signals the magnitude of whichvaries depending on where the received pulse of energy intercepts thepatterns. The ratio of one signal to the other varies exponentiallyaccording to the direction of the received signal. The crystal detectorsand logarithmic amplifiers take the logarithm of this signal and theresult is a logarithmic signal which varies in linear relation to thereceived signal.

This can be seen more clearly by equation representation. Let:

E Voltage of one antenna in a pair of complementary antennas, such asantenna pair 32, 34 or pair 36, 38 I E Voltage of the other antenna inthe pair e and k= Constants 0= Angle of arrival of the receivedradiation.

The antennas have patterns such that:

the log amplifiers take the log s of the above system,

10 E /E =log e 2 which is equivalent to:

log E -log E =k9 3 so that by subtracting two quantities, log 6 E, andlog E E a signal is produced which varies linearly with received signaldirection, 0, and is independent of signal level since a ratio, log E E/E is being taken. Although the relationships between the antenna,crystal detectors, and amplifiers are not exactly as represented bythese equations, the principle is well illustrated by the use of theequations.

Signals from each of the logarithmic periodic antenna elements 32 38 arecrystal detected and amplified by logarithmic video amplifiers 32d 38dfor use by the system. The transfer characteristic ofeach amplifierobtains a voltage analog of the logarithm of the amplitude of the signalinput to the crystal detector. A block diagram of a single logarithmicvideo amplifier is shown in FIG. 9 and a schematic diagram is shown inFIG. 10. The following explanation of amplifier operation will be madewith reference to these two figures.

Signals from each crystal detector, in this case detector 32c, arecoupled to the input of emitter follower 80 by capacitor 79 andresistors 79a, 79b and 79c in order to preserve the crystal biascurrent, 90 microamperes. The first emitter follower 80 drives a secondemitter follower 82 which in turn drives a grounded base amplifier 84.The grounded base amplifier 84 is decoupled from its load by a thirdemitter follower 86. The sigial at the emitter of the first emitterfollower 80 is divided by resistors 88, 90 and the signal at the emitterof the third emitter follower 86 is divided by means of re sistors 92,94 to obtain four output signals of the same polarity and with thesignals attenuated in the ratio of approximately 1:10:10021000.Transistors 102," 100, 96, 98 are all operated in their logarithmicranges and due to the fact that signals are attenuated to theseresistors in the ratio of 1:10:10011000, respectively, a wide amplituderange of input signals can be amplified logarithmically.

The attenuation ratios would not be exactly as stated because the inputnetworks to the logarithm taking transistors would not be identical andbecause the detection characteristic of the crystal changes from squarelaw to linear in the region of -12 db for signal strength, and thistransition is adjusted for by adjusting resistors 88, 90, 92 and 94 andthe 20K resistors in the emitter circuits of the transistors 96, 98,100, 102.

The signals from the four tap points of resistors 88, 90, 92 and 94 areapplied to the bases of four log-taking transistors 96, 98, 100, 102respectively and the outputs of transistors 96, 98, 100 and 102 aresummed in the resistor 104 which is common to the collectors of thesetransistors. These four log-taking transistors are operated at a lowemitter current level where the transfer characteristics are nonlinear.When the operation of transistor 102 is in the level off region in whichsubstantially the same output exists for any input, transistor 100 takesover and operates in its logarithmic range until it is in its level offregion, after which transistor 96 takes over etc.

In FIG. a, where the output of a typical log-taking transistor isplotted against the input, the curve is logarithmic to the level offpoint 103. Essentially no signal is emitted from the transistor outputafter this point because the amplifier output is coupled by capacitor105 to the output coaxial cable 107, and will not pass d-c signals.Input pulses before the level off point 103 are passed because they havea predetermined rise time which in the pre-level off portion results ina corresponding rise time that the capacitor 105 will pass.

At high signal levels, the base-emitter junction of the first log-takingtransistor 102, which contributes the major portion of collector currentchanges at low signal levels, would be damaged expect for the presenceof a diode 106 in series with the base of this transistor. The diode 106limits the current to the base of transistor 102 since the diode outputlevels off after input reaches a predetermined point.

A difficulty brought out by a time delay of approximately 20millimicroseconds existing between signals from log-taking transistors100 and 102 and log-taking transistors 96 and 98 as a result of theaddition of three more transistors in the signal path to the first pair,was corrected by the addition of a 11' section delay network 109 betweenthe collectors of the second pair of logtaking transistors and thecommon collector summing resistor 104 to which the first pair aredirectly connected. Also, since the detection characteristic of crystal32c changes from square law to linear as the crystal output increases,the quiescent current of logtaking transistor 98 as well as the basedrive signal ratios at the bases of log-taking transistors 96 and 98must be modified in order to realize the desired over-all transfercharacteristic. Emitter follower 101 is to be used to unload resistor104 from the circuit to the right thereof.

After summing the currents from the four log-taking transistors, thesignal is' amplified by amplifier 108 and inverted to provide a largepulse (0.5 to 2.5 volts in this embodiment) compatible with therequirements of the system. The cable which transmits the pulse to thesystem is matched to the amplifier 108 by a cascaded emitter followerpair 1 l0 and 112.

The alignment of the logarithmic video amplifiers is accomplished withthe aid of a swept amplitude signal generator, (FIG. 11). AHewlett-Packard model 616A UHF Signal Generator 1 14 has a cutoffattenuator shaft 116 which is connected to rack 118 which is engagedwith and reciprocated by oscillating pinion 120.

A cutoff attenuator is a coaxial transmission line which has the centerconductor severed and with the ends movable toward and away from oneanother to attenuate the signal in an exponential manner. This is morefully set forth in Techniques of Microwave Measurements, Montgomery Vol.1 l of Radiation Laboratory Series, McGraw-I-Iill, p. 685 et seq.

Pinion 120 is oscillated by crank 122 which is pivoted to a point on thecircumference of motor driven disc 124. Amplitude of the signal fromgenerator 114 is controlled according to the position of cutoffattenuator 116.

The pointers 126, 128 of two precision linear potentiometers 130, 132are attached to the rack 118 and reciprocate along the referencepotentiometers 130, 132 respectively. The voltage from the firstpotentiometer 130 is a reference voltage and varies in a linear relationto the position of rack 118 and is compared with the amplifier output asdescribed below, and the voltage from the second potentiometer 132provides horizontal deflection voltage for an oscilloscope 136 display.Signals to be compared are plugged into inputs A & B.

Input signals to the crystal detectors 32c, 34c and corresponding logvideo amplifiers 32d and 34d which are to be aligned, are obtained bydividing the power from the signal generator 114 using a 3 decibelbroadband directional coupler 134. In this fashion two equal, sweptamplitude signals are available. The signals can be made unequal byinserting a fixed amount of the desired attenuation in one arm of thedirectional coupler.

Amplifier alignment of each amplifier is accomplished by comparing onoscilloscope 136 the output of a logarithmic video amplifier which isbeing aligned and the output of the reference potentiometers 130, 132. Alog video amplifier is properly adjusted and matched to the crystaldetector when the difference between the amplifier output and thepotentiometers 130, 132 voltage is zero or very small. The secondpotentiometer 132 output as mentioned, provides the horizontaldeflection or sweep voltage. Therefore, the voltage level at which anydiscrepancy between the amplifier output and the reference output takesplace can easily be seen by noting at which horizontal position it takesplace.

When both amplifiers have been compared with the referencepotentiometers 130 and 132, they may be compared to each other byadjusting until the difference between the two amplifier output signalsis minimized as the input signal power is swept periodically.

After the amplifiers 32d 34d have processed the signals supplied them,the signals are fed to a sum and difference device 44, which takes thedifference of the two logarithm signals, which is the quotient of thesignals, and feeds this to a boxcar circuit 50. Device 44 also takes thesum of these two signals and feeds this to a second summing device 48.Boxcar circuit 50 is connected to one pair of deflection plates of anindicator 54. The boxcar circuit 50, which is operated by a leading edgegate 66 to receive only the leading edge of a pulse, stores theamplitude of the leading edge of each pulse which is received by it sothat an optimum display is presented on the screen (not shown) ofindicator 54.

The signals from vertically aligned antennas 36 and 38 are fed from logamplifiers 36d and 38d to a sum and difference device 56, which takesthe difference of the two logarithm signals or quotient, and feeds thisto a boxcar circuit 60 and the sum to summing device 48. Boxcar circuit60 performs the same function as circuit 50 and is connected to a secondset of deflection plates of indicator 54.

The output of the summing device 48 is connected to a signal presencedevice or threshold 64, which in turn actuates a leading edge gate pulsegenerator 66. The threshold device 64 will not pass signals to actuategenerator 66, which are below a predetermined minimum so that the veryweak signals, such as crystal noise signals, the signals which fall outof the dynamic logarithmic range of the log amplifiers 32c 380, will notbe passed since gate 66 will not operate boxcars 50, 60.

An amplitude selector 68 is connected to amplifiers 32d 38d, and isconnected through a squelch circuit 69 to an and gate 71 which isconnected between the leading edge gate generator 66 and a blockingoscillator 72 which controls the intensity power to indicator 54.Oscillator 72 is also connected to boxcar circuits 50 and 60 todetermine the pulse length of signals passing to indicator 54 byresetting the boxcar detector output to zero immediately after display.A power level switch 70 is connected between amplitude selector 68 andeach attenuator 32b 38b. If any antenna receives a signal exceeding apredetermined maximum, then the amplitude selector will send a signal tothe switch 70 which will adjust attenuators 32b 38b to correspondinglyincrease signal attenuation. If a signal comes in which is so strongthat it will be outside the attenuation range and hence, the log rangeof amplifiers 32c 38c, squelch circuit 69 will cut off its signal to andgate 71 which will, in turn, cut off a signal to blocking oscillator 72and intensity to indicator 54, thus disabling the system.

If the signal received by signal presence device 64 from summing device48 exceeds a predetermined minimum, indicating that it is an acceptablesource and not a noise signal, and if all of the signals from amplifiers32d 38d are less than a predetermined maximum, indicating that signal iswithin the log range of amplifiers 32d 38d, the leading edge gategenerator 66 is actuated, operating boxcar circuits 50 and 60 so thatthey detect the leading edge of the pulses from circuits 44 and 56respectively, to provide signals to deflection plates of indicator 54.

The manner in which leading edge gate generator 66 minimizes reflectedsignals from appearing on the display means will nowbe discussed inconnection with FIGS. 7 and 8. Signals from a source, such as source 80,will tend to be reflected off the ground or other objects, as shown inFIG. 7 and therefore, be received by vehicle 26 at a different anglethan the direct path signal and give an erroneous signal or signals tothe pilot. In order to minimize this, only the leading edge of thepulses radiated by a source is used to establish the display ofradio-frequency sources on the cathode ray tube screen. A reflected pathsignal such as waveform r, would arrive at the vehicle 26 later than thedirect path radiation, waveform d, since the reflected path is longer,generator 66 will energize the boxcars 50 and 60 for only a very shortinterval, as indicated by the letter i" in FIG. 8, and the reflectedpath signal will not be considered in determining the voltages of thecathode ray tube deflection plates 52, 62. Therefore, the reflected pathsignals will be minimized. The rise time of the pulses going to theleading edge gate should be steep enough so that the full pulse heightis obtained before reflections start to interfere.

An audio circuit 74 may be connected to oscillator 72 to indicateaurally to the operator the repetition frequency of signal pulses.

The source may radiate horizontal, vertical or circular polarizedsignals and if so the antennas would be aligned at 45 to the horizontalin order to receive all these polarizations of signal; Therefore, if thevehicle rolls about its longitudinal axis during a signal reading,signals may come in much stronger on one set of antennas, which isaligned with polarization than in the other set which is not alignedwith this polarization. To prevent disability to one of the orthogonalmeasurement channels during this condition, a connection is made betweena diflerence output of device 48 and and gate 71 through roll cutoutdevice 49. If the difference between the inputs to device 48 exceeds amaximum, the and gate is controlled by device 49 to prevent a signalfrom being transmitted to indicator 54.

OPERATION Briefly, the operation of the receiver in this system is asfollows:

A signal is received by antennas 32 38 passed through attenuator 32b38b, detected by rectifiers 32c 38c, removing the high frequency carrierportion, and amplified by log amplifiers 32d 38d. Antennas 32 and 34 maybe horizontally aligned, and signals from these two antennas are sent bylogarithmic amplifiers 32d and 34d, respectively, to a sum anddifference device 44, which delivers the logarithm difference orquotient of the two signals, which is in linear relation to the receivedsignal direction, to boxcar circuit 50 and the log sum to summing device48. In like manner, the sum and difference of the signals fromvertically aligned antennas 36 and 38 are delivered by sum anddifference device 56 to summing device 48 and boxcar circuit 60respectively. The characteristics of amplifiers 32d- 38d must be closelydesigned or mated to the crystal characteristics of the detectors 32c38c and the antenna lobe patterns since the horizontal and verticalposition of a signal source is determined by the points at which asignal intercepts the lobe patterns, and the accuracy of the system isdetermined by the combined characteristics of the antennas, crystals andlog amplifiers. V

Boxcar circuits 50 and 60 send signals to deflection plates of indicator54 to display the angular coordinates of the signal source on the screenof indicator 54. In order not to reproduce noise signals, which are oflower amplitude than the source signals, devices 44 and 56 also sendsignals to a sum and difference device 48 which sends the sum of the twosignals to a signal presence or threshold device 64 which allows asignal to pass to leading edge gate generator 66 only if their sum isabove a predetermined minimum.

Also, to attenuate or prevent very strong signals, which would beoutside the log range of amplifiers 32d 38d, the signals are passed toamplitude selector 68 which sends a signal to switch 70 which acts tooperate attenuators 32b 38b to attenuate the signals until they fall inthe range of the logarithmic amplifiers.

If a signal greater than a predetermined maximum is received by anyantenna, this signal will pass through the device 68 which will thenoperate squelch circuit 69 to squelch the signals.

Threshold device 64 sends it signal to a leading edge gate generator 66.Leading edge gate 66 operates boxcar circuits 50 and 60 for a very shortperiod so that only the amplitude near the leading edges of thedifference pulses are allowed to pass to deflection means of indicator54. This prevents ground reflections which arrive at antennas 32 38later than the direct radiations from the radio-frequency sources frominfluencing the source location presented on the indicator 54.

Also gate generator 66 provides a signal to and gate 71 which alsoreceives a signal from device 49 which is utilized in the case ofpolarized signals, if the vehicle roll is not excessive, and fromsquelch circuit 69 if the predetermined maximum has not been exceeded.If all the signals to and" gate 71 are present, oscillator 72 will beactuated to send signals to boxcar circuits 50 and 60 to limit the pulselengths therein, and to indicator 54 to supply intensity power. Audiocircuit 74 is used to determined the nature and repetition frequency ofpulses to help in identifying the source signal.

By measuring the inputs to indicator 54 and comparing them with areference set of measurements which correspond to a desired line oftravel for the vehicle 26, an error signal, which is the differencebetween its measured inputs and the reference values, can be used tobring and maintain the vehicle on the desired line of travel.

Although this invention has been disclosed and illustrated withreference to particular applications, the principles involved aresusceptible to numerous other applications, which will be apparent topersons skilled in the art. The invention is, therefore, to be limitedonly as indicated by the scope of the appended claims.

Having thus described our invention, we claim:

1. An apparatus comprising:

means for receiving a signal and providing an output signal non-linearlyrelated to received signal strength,

attenuating means having an input connected to said signal receivingmeans and a plurality of outputs, said attenuating means for attenuatingsaid receiving means output signal different amounts at each of saidoutputs,

a plurality of function taking circuits having a common output, eachhaving an input connected to respective ones of said attenuating meansoutputs; and

means individually connected to each of said function taking circuitsfor independently controlling the transfer characteristics of each ofsaid circuits to compensate for the nonlinearly related receiving meansoutput signal thereby providing a preselected function of said receivedsignal at said common output regardless of received signal strength.

2. The apparatus of claim 1 wherein said attenuating means comprises:

a plurality of resistors of preselected different values being seriesconnected to said signal receiving means.

3. The apparatus of claim 1 wherein said function taking circuitscomprise:

a transistor having a control terminal connected to a respective one ofsaid attenuating means outputs, an output terminal connected to saidcommon output, and a third terminal connected to said last means.

4. The apparatus of claim 3 wherein said transistor is an NPN transistorin common emitter configuration such that said control terminal is thebase terminal, said output terminal is the collector terminal, and saidthird terminal is the emitter terminal.

5. The apparatus of claim 3 wherein said last means comprises:

means to control the output bias level of said transistor.

6. The apparatus of claim 5 wherein said signal receiving means includea detector for providing an output signal which is nonlinearly relatedto received signal strength and wherein said output bias level of saidtransistor is selected within the nonlinear range of said transistor tocompensate for the nonlinearity of said detector.

7. An apparatus comprising:

means for receiving a signal and providing an output signal which isnonlinearly related to received signal strength,

attenuating means having an input connected to said signal receivingmeans and a plurality of outputs, said attenuating means for attenuatingthe output signal therefrom in substantially logarithmically relatedamounts at each of said outputs,

a plurality of function taking circuits having a common output, eachhaving an input connected to respective ones of said attenuating meansoutputs; and

means individually connected to each of said func tion taking circuitsfor independently controlling the transfer characteristics of each ofsaid circuits to compensate for the nonlinearly related receiving meansoutput signal thereby providing a logarithmic function of said receivedsignal at said common output regardless of received signal strength.

8. An apparatus for receiving signals comprising:

an antenna for receiving a signal,

a detector connected to said antenna for providing a voltagenon-linearly related to said signal,

attenuating means having an input connected to said detector and aplurality of outputs, said attenuating means for attenuating said outputvoltage difl5 ferent amounts at each of said outputs,

a plurality of function taking circuits having a common output, eachhaving an input connected to respective ones of said attenuating meansoutputs; and

means individually connected to each of said function taking circuitsfor independently controlling the transfer characteristics of each ofsaid circuits to compensate for the detector nonlinearity.

9. The apparatus of claim 8 further including:

a second antenna for receiving said signal,

a second detector connected to said second antenna for providing asecond voltage nonlinearly related to said signal,

a second attenuating means having an input connected to said seconddetector and a plurality of outputs, said attenuating means forattenuating said output voltage different amounts at each of saidoutputs,

a plurality of second function taking circuits having a second commonoutput, each having an input connected to respective ones of said secondattenuating means outputs; and

second means individually connected to each of said second functiontaking circuits for independently controlling the transfercharacteristics of each of said second function taking circuits tocompensate for said second detector nonlinearity.

10. The apparatus of claim 9 including: further including:

means connected to said common outputs for comparing the output signaltherefrom and generating a signal representative of the difference ofsaid common output signals.

1 l. The apparatus of claim 10 wherein said attenuating amounts of saidfirst and second attenuating means and said transfer characteristics ofsaid first and second function taking circuits are such that said firstand second common output signals are logarithmic functions of said firstand second received signals respectively.

1. An apparatus comprising: means for receiving a signal and pRovidingan output signal nonlinearly related to received signal strength,attenuating means having an input connected to said signal receivingmeans and a plurality of outputs, said attenuating means for attenuatingsaid receiving means output signal different amounts at each of saidoutputs, a plurality of function taking circuits having a common output,each having an input connected to respective ones of said attenuatingmeans outputs; and means individually connected to each of said functiontaking circuits for independently controlling the transfercharacteristics of each of said circuits to compensate for thenonlinearly related receiving means output signal thereby providing apreselected function of said received signal at said common outputregardless of received signal strength.
 1. An apparatus comprising:means for receiving a signal and pRoviding an output signal non-linearlyrelated to received signal strength, attenuating means having an inputconnected to said signal receiving means and a plurality of outputs,said attenuating means for attenuating said receiving means outputsignal different amounts at each of said outputs, a plurality offunction taking circuits having a common output, each having an inputconnected to respective ones of said attenuating means outputs; andmeans individually connected to each of said function taking circuitsfor independently controlling the transfer characteristics of each ofsaid circuits to compensate for the nonlinearly related receiving meansoutput signal thereby providing a preselected function of said receivedsignal at said common output regardless of received signal strength. 2.The apparatus of claim 1 wherein said attenuating means comprises: aplurality of resistors of preselected different values being seriesconnected to said signal receiving means.
 3. The apparatus of claim 1wherein said function taking circuits comprise: a transistor having acontrol terminal connected to a respective one of said attenuating meansoutputs, an output terminal connected to said common output, and a thirdterminal connected to said last means.
 4. The apparatus of claim 3wherein said transistor is an NPN transistor in common emitterconfiguration such that said control terminal is the base terminal, saidoutput terminal is the collector terminal, and said third terminal isthe emitter terminal.
 5. The apparatus of claim 3 wherein said lastmeans comprises: means to control the output bias level of saidtransistor.
 6. The apparatus of claim 5 wherein said signal receivingmeans include a detector for providing an output signal which isnonlinearly related to received signal strength and wherein said outputbias level of said transistor is selected within the nonlinear range ofsaid transistor to compensate for the nonlinearity of said detector. 7.An apparatus comprising: means for receiving a signal and providing anoutput signal which is nonlinearly related to received signal strength,attenuating means having an input connected to said signal receivingmeans and a plurality of outputs, said attenuating means for attenuatingthe output signal therefrom in substantially logarithmically relatedamounts at each of said outputs, a plurality of function taking circuitshaving a common output, each having an input connected to respectiveones of said attenuating means outputs; and means individually connectedto each of said function taking circuits for independently controllingthe transfer characteristics of each of said circuits to compensate forthe nonlinearly related receiving means output signal thereby providinga logarithmic function of said received signal at said common outputregardless of received signal strength.
 8. An apparatus for receivingsignals comprising: an antenna for receiving a signal, a detectorconnected to said antenna for providing a voltage non-linearly relatedto said signal, attenuating means having an input connected to saiddetector and a plurality of outputs, said attenuating means forattenuating said output voltage different amounts at each of saidoutputs, a plurality of function taking circuits having a common output,each having an input connected to respective ones of said attenuatingmeans outputs; and means individually connected to each of said functiontaking circuits for independently controlling the transfercharacteristics of each of said circuits to compensate for the detectornonlinearity.
 9. The apparatus of claim 8 further including: a secondantenna for receiving said signal, a second detector connected to saidsecond antenna for providing a second voltage nonlinearly related tosaid signal, a second attenuating means having an input connected tosaid second detector and a plurality of outputs, said attenuating meansfor attenuating said output voltage different amounts at each of saidoutputs, a plurality of second function taking circuits having a secondcommon output, each having an input connected to respective ones of saidsecond attenuating means outputs; and second means individuallyconnected to each of said second function taking circuits forindependently controlling the transfer characteristics of each of saidsecond function taking circuits to compensate for said second detectornonlinearity.
 10. The apparatus of claim 9 including: further including:means connected to said common outputs for comparing the output signaltherefrom and generating a signal representative of the difference ofsaid common output signals.